Rotation angle detecting apparatus

ABSTRACT

A rotation angle detecting apparatus comprises a shaft portion space formed in a rotation shaft, a bearing holder space, a first condenser lens, a second condenser lens to face the first condenser lens, a detection pattern provided at a focal position of one of the first condenser lens and the second condenser lens, an image sensor provided at a focal position of the other of the first condenser lens and the second condenser lens, and an arithmetic unit for calculating an angle displacement of the rotation shaft. The arithmetic unit carries out a total circumferential scanning, extracts a frequency component, carries out a scanning of a reference designation pattern, and calculates the angle displacement of the rotation shaft based on a phase difference of the frequency component and the number of frequencies corresponding to a change in a position of the reference designation pattern.

BACKGROUND OF THE INVENTION

The present invention relates to a rotation angle detecting apparatusthat highly accurately detects a rotation angle.

In the industrial machines, the surveying instruments, and others, arotation angle detecting apparatus that detects a rotation angle isprovided in a rotation unit. Nowadays, a highly accurate and smallrotation angle detecting apparatus is demanded for these machines orinstruments.

In general, as a rotation angle detecting apparatus used in a surveyingequipment, an encoder is adopted, and a highly accurate encoder isexpensive. Further, in order to improve a rotation accuracy of arotation shaft to a required accuracy, just managing a machiningaccuracy for each individual component is difficult, the fine adjustmentand the precision finishing in an assembled state of the rotation shaftand a bearing holder are required, which results in a very high price.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a rotation angledetecting apparatus that is small in size and has a simpleconfiguration.

To attain the above object, a rotation angle detecting apparatusaccording to the present invention comprises a shaft portion spaceformed in a rotation shaft, a bearing holder space formed in a bearingholder, a first condenser lens accommodated in the shaft portion space,a second condenser lens provided in the bearing holder space andprovided to face the first condenser lens, a detection pattern providedat a focal position of one of the first condenser lens and the secondcondenser lens, an image sensor provided at a focal position of theother of the first condenser lens and the second condenser lens, and anarithmetic unit which calculates an angle displacement of the rotationshaft based on a signal from the image sensor, wherein the detectionpattern has an angle detection pattern in which the bar-like linesegments extending in a radial direction are arranged at a predeterminedangle pitch and a ring-like track is formed, and a reference designationpattern indicates a reference position of the angle detection pattern,and the arithmetic unit carries out a total circumferential scanning onthe track with a predetermined radius with a center of the angledetection pattern as the center in regard to a projection image of thedetection pattern obtained by the image sensor, extracts a frequencycomponent, scans the reference designation pattern, and calculates theangle displacement of the rotation shaft based on a phase difference ofthe frequency component and the number of frequencies corresponding to achange in position of the reference designation pattern.

Further, in the rotation angle detecting apparatus according to thepresent invention, the line segments of the angle detection pattern areobtained by dividing the track by 2n radii at an equal angle pitch andhave a wedge-like shape.

Furthermore, in the rotation angle detecting apparatus according to thepresent invention, the detection pattern has a circular pattern that isconcentric with the angle detection pattern, and the arithmetic unitscans a projection image of said circular pattern in two directionsorthogonal to each other and obtains the center of the angle detectionpattern.

Moreover, in the rotation angle detecting apparatus according to thepresent invention, the track is equally divided into an even number, anaverage of the phases of the frequency components of the respective linesegments in each divided portion is obtained, and the center of theangle detection pattern is detected from a phase difference between thedivided portions that face each other.

Additionally, in the rotation angle detecting apparatus according to thepresent invention, the total circumferential scanning is carried outmore than once with different radii.

According to the present invention, the rotation angle detectingapparatus comprises a shaft portion space formed in a rotation shaft, abearing holder space formed in a bearing holder, a first condenser lensaccommodated in the shaft portion space, a second condenser lensprovided in the bearing holder space and provided to face the firstcondenser lens, a detection pattern provided at a focal position of oneof the first condenser lens and the second condenser lens, an imagesensor provided at a focal position of the other of the first condenserlens and the second condenser lens, and an arithmetic unit forcalculating an angle displacement of the rotation shaft based on asignal from the image sensor, wherein the detection pattern has an angledetection pattern in which bar-like line segments extending in a radialdirection are arranged at a predetermined angle pitch and a ring-liketrack is formed by bar-like line segments, and a reference designationpattern for indicating a reference position of the angle detectionpattern, and the arithmetic unit carries out a total circumferentialscanning on the track with a predetermined radius with a center of theangle detection pattern as the center in regard to a projection image ofthe detection pattern obtained by the image sensor, extracts a frequencycomponent, carries out a scanning of the reference designation pattern,and calculates the angle displacement of the rotation shaft based on aphase difference of the frequency component and the number offrequencies corresponding to a change in position of the referencedesignation pattern. Therefore, the distortion of the pattern, a shapeerror of the line segments, the illumination unevenness, a difference insensitivity between the pixels, and a quantization error are canceledout and averaged, and hence a small and highly accurate configurationcan be obtained.

Further, according to the present invention, since the line segments ofthe angle detection pattern are obtained by dividing the track by 2nradii at an equal angle pitch and have a wedge-like shape, the equalsignals can be acquired as the signals obtained by scanning the wholecircumference irrespective of a difference in radius.

Further, according to the present invention, since the detection patternhas a circular pattern that is concentric with the angle detectionpattern, and the arithmetic unit scans a projection image of thecircular pattern in two directions orthogonal to each other and obtainsthe center of the angle detection pattern, the center position can beeasily detected by the signal processing without physically measuringthe center, and hence the center can be measured at an arbitrary timepoint, e.g., during the measurement.

Further, according to the present invention, the track is equallydivided into an even number, an average of the phases of the frequencycomponents of the respective line segments in each divided portion isobtained, and the center of the angle detection pattern is detected froma phase difference between the divided portions that face each other.Therefore, the distortion of the pattern, a shape error of the linesegments, and others are averaged, and the center position can be highlyaccurately detected.

Furthermore, according to the present invention, since the totalcircumferential scanning is carries out more than once with differentradii, influences of the distortion of the pattern, a shape error of theline segments, and the illumination unevenness are further averaged, thedifferent pixels are used, a quantization error such as a difference insensitivity between the pixels can be averaged and accuracy is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a rotation angle detectingapparatus according to an embodiment of the present invention;

FIG. 2 is a schematic block diagram of an arithmetic unit according tothe present embodiment;

FIG. 3 is a view showing an example of a detection pattern used in thepresent embodiment;

FIG. 4 is a flowchart of the angle detection and the center detectionaccording to the present embodiment;

FIG. 5 is a flowchart of the rotation angle detection according to thepresent embodiment;

FIG. 6 is an explanatory view showing a state of the scan when the angledetection using the detection pattern is performed;

FIG. 7A and FIG. 7B are the waveform charts of the signals obtained bythe detection pattern, where FIG. 7A shows the signals from an angledetection pattern and FIG. 7B shows the signals from a referencedesignation pattern; and

FIG. 8 is an explanatory view in case of obtaining a center positionusing the angle detection pattern.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment according to the present invention will now be describedhereinafter with reference to the accompanying drawings.

First, referring to FIG. 1, a rotation angle detecting apparatusaccording to an embodiment of the present invention will be explained.

In FIG. 1, a reference numeral 1 denotes a rotation shaft which is arotation angle measurement target, and the rotation shaft 1 is rotatablysupported by a bearing holder 3 through a bearing 2.

At an end portion of the rotation shaft 1, a cylindrical shaft portionspace 4 is formed so as to be concentric with a shaft center of therotation shaft 1, and a shaft end portion has a hollow configuration. Abearing holder space 5 is formed in the bearing holder 3 on an extendedline of the shaft center of the shaft portion space 4. The bearingholder space 5 is a space that is concentric with the shaft portionspace 4 and continuous with the shaft portion space 4. In the shaftportion space 4 and the bearing holder space 5, the primary constituentelements of a rotation angle detecting apparatus 6 are accommodated.

A first condenser lens 7 is provided in the shaft portion space 4, and asecond condenser lens 8 is provided in the bearing holder space 5. Amagnifying power of each of the first condenser lens 7 and the secondcondenser lens 8 is a magnifying power of 1, and these lenses have thesame focal length.

The first condenser lens 7 and the second condenser lens 8 have opticalaxes 9 a and 9 b, respectively, the optical axis 9 a substantiallycoincides with a shaft center of the rotation shaft 1, and the opticalaxis 9 b substantially coincides with a shaft center of the bearingholder space 5.

On a bottom portion of the shaft portion space 4, a detection pattern 11is provided, and the detection pattern 11 is placed at a focal positionof the first condenser lens 7. Further, an image sensor 12 is providedin the bearing holder space 5, and the image sensor 12 is placed at afocal position of the second condenser lens 8. It is preferable for theimage sensor 12 to be held so that the thermal strain does not occureven though a temperature increases.

A light emitting unit to illuminate the detection pattern 11 is providedat appropriate position in the bearing holder space 5 or in the shaftportion space 4. The drawing shows a light emitting unit 13 that isprovided on a bottom portion of the shaft portion space 4 andilluminates the detection pattern 11 as an example.

A basic shape of the detection pattern 11 is a circular shape, and adiameter of the pattern is approximately 5 mm to 10 mm. Furthermore, itis preferable for a material of a member forming the detection pattern11 to be equal or equivalent to a material of the rotation shaft 1 and amaterial of the bearing holder 3, or have the same or equivalent thermalexpansion coefficient as thermal expansion coefficient of the materialof the bearing holder 3.

As the image sensor 12, a CCD or a CMOS sensor or the like which is anaggregate of the pixels is used. The optical axis 9 b is set so as torun through an origin of a coordinate system assumed for the imagesensor 12, and a position (a coordinate position) of each pixel can bespecified on the image sensor 12.

A photodetection signal from the image sensor 12 is input to anarithmetic unit 14, and the arithmetic unit 14 is configured to measurea rotation angle of the rotation shaft 1 and a runout caused due to aninclination (an inclination angle) of the rotation shaft 1 based on thephotodetection signal.

The detection pattern 11, the first condenser lens 7, the secondcondenser lens 8, the image sensor 12, and others accommodated in theshaft portion space 4 and the bearing holder space 5 constitute aprimary portion of the rotation angle detecting apparatus 6. Further,the first condenser lens 7 and the second condenser lens 8 constitute arotation angle detection optical system 10 for forming a projectionimage of the detection pattern 11 on the image sensor 12.

It is to be noted that the detection pattern 11 may be provided in thebearing holder space 5, and the image sensor 12 may be provided in theshaft portion space 4.

As shown in FIG. 2, the arithmetic unit 14 is mainly configured with asignal processing unit 16, an arithmetic control unit 17, an angledetection unit 18, a runout detection unit 19, a storage unit 20, andothers.

The signal processing unit 16 executes the signal processing, e.g.,amplification or A/D conversion with respect to data output from theimage sensor 12 so that the data can be stored.

In the storage unit 20, various programs are stored. The programsinclude a control program for controlling the acquisition of a signalfrom the image sensor 12, a rotation angle arithmetic program fordetecting a rotation angle of the rotation shaft 1, a runout arithmeticprogram for detecting a runout of the rotation shaft 1, a signalprocessing program for executing signal processing, e.g., extracting asignal required to detect a rotation angle or detect a runout, andothers. Furthermore, in the storage unit 20, the image data output fromthe image sensor 12, the data obtained by calculation (which will bedescribed later), and others are stored.

The arithmetic control unit 17 performs calculating and controllingbased on the program, carries out a synchronization control for theacquisition of a signal from the image sensor 12, and executes theprogram.

The angle detection unit 18 calculates a rotation angle of the rotationshaft 1 based on a signal from the image sensor 12, and the angledetection unit 18 is mainly configured with the rotation anglearithmetic program and the arithmetic control unit 17. Moreover, therunout detection unit 19 calculates a runout of the rotation shaft 1based on a signal from the image sensor 12, and the runout detectionunit 19 is mainly configured with the runout arithmetic program and thearithmetic control unit 17.

An example of the detection pattern 11 used in the present embodimentwill now be described with reference to FIG. 3.

A basic shape of the detection pattern 11 is a circle, and the center ofthe detection pattern 11 is configured to substantially coincide with anoptical axis of the first condenser lens 7, i.e., the optical axis 9 a.

The detection pattern 11 is constituted of a circular pattern 25 whichis a centering pattern provided at the center, and an angle detectionpattern 26 and a reference designation pattern 27 which are arranged aspatterns for angle detection around the circular pattern 25concentrically with the circular pattern 25. The circular pattern 25consists of a plurality of perfect circles (two concentric multi circlesin the drawing) drawn with a predetermined line width. It is to be notedthat, as the centering pattern, a pattern that enables determining thecenter can suffice, and for example, a cross line can be used.

The angle detection pattern 26 has a configuration that n line segments26 a (which are darkened portions in the drawing) with a predeterminedlength extending in a radial direction are arranged on the entirecircumference at an equal angle pitch, and the line segments 26 a form aring-like track. In other words, the angle detection pattern 26 isformed by the fact that a ring having a predetermined track width (apredetermined length in the radial direction) is equally divided into 2non the entire circumference by 2n radii, and the line segments 26 a areformed every other row. Therefore, each of the line segments 26 a has awedge-like shape and also has a center angle α of 360°/2n. Additionally,the center of the angle detection pattern 26 is the same as the centerof the circular pattern 25.

The reference designation pattern 27 is formed on the inner side of theangle detection pattern 26 and concentric with the angle detectionpattern 26. The reference designation pattern 27 has an arc shape havinga predetermined width in the radial direction. Further, the referencedesignation pattern 27 is divided into a plurality of patterns in thecircumferential direction and formed of one position designation pattern27 a and a pair of direction designation patterns 27 b arranged on boththe sides of the position designation pattern 27 a.

The position designation pattern 27 a has the same center angle as theline segments 26 a, and the position designation pattern 27 a is placedon the same radial line as one of the line segments 26 a.

Each of the direction designation patterns 27 b has a symmetricalposition and a symmetrical shape with respect to the positiondesignation pattern 27 a, and also has a circumferential length (acenter angle 5 a) across the three line segments 26 a. It is to be notedthat a width (a circumferential length) of each of the directiondesignation patterns 27 b is not restricted to the width correspondingto the three line segments 26 a, but a width different from the width ofthe line segment 26 a can suffice.

If the detection pattern 11 is a reflection type, the line segments 26a, the position designation pattern 27 a, and the direction designationpatterns 27 b may be designed to not reflect the light and any otherportion may be designed to be reflective, or the line segments 26 a, theposition designation pattern 27 a, and the direction designationpatterns 27 b may be designed to be reflective and any other portion maybe designed to be non-reflective.

Alternatively, if the detection pattern 11 is of a transmissive type,the line segments 26 a, the position designation pattern 27 a, and thedirection designation patterns 27 b may be designed to transmit thelight therethrough and any other portion may be designed to benon-transmissive, or the line segments 26 a, the position designationpattern 27 a, and the direction designation patterns 27 b may bedesigned to be non-transmissive and any other portion may be designed tobe transmissive.

In the following description, the detection pattern 11 is of thetransmissive type and the line segments 26 a, the position designationpattern 27 a, and the direction designation patterns 27 b are of thenon-transmissive type.

An operation of the rotation angle detecting apparatus 6 will now bedescribed.

The rotation angle detecting apparatus 6 can detect a rotation angle anda runout (a slant of the rotation shaft) caused by the rotation.

An image of the detection pattern 11 is projected onto the image sensor12 in the one-on-one relationship by the operation of the firstcondenser lens 7 and the second condenser lens 8, and the image sensor12 produces a signal corresponding to the image of the detection pattern11 as received.

When the rotation shaft 1 rotates, the detection pattern 11 rotatesintegrally with the rotation shaft 1, and the detection pattern image asrotated is projected onto the image sensor 12. Since the image sensor 12produces a photodetection signal in accordance with each pixel, forexample, when the angle detection pattern 26 and the referencedesignation pattern 27 move, a position of each pixel that photodetectsthe images of the angle detection pattern 26 and the referencedesignation pattern 27 varies. Therefore, detecting a change in positionof each pixel that photodetects the images of the angle detectionpattern 26 and the reference designation pattern 27 based on the signalfrom the image sensor 12 enables detecting a rotation angle of therotation shaft 1 with respect to the bearing holder 3.

Moreover, when the rotation angle is differentiated with a timedifference between the angles before and after the angle varies, arotating speed can be detected. Additionally, when a change in centerposition of the angle detection pattern 26 is detected, a runout can bedetected with this time difference.

With reference to FIG. 4 to FIG. 6, the detection of a rotation angleand the detection of a runout will now be described.

First, the initial data is acquired at STEP 01 to STEP 08.

STEP 01 A signal from the image sensor 12 is acquired before themeasurement starts (i.e., an initial state), an image data of thedetection pattern 11 (which will be referred to as a pattern image datahereinafter) is acquired through the signal processing executed by thesignal processing unit 16, and the pattern image data is stored in thestorage unit 20.

In regard to each of the stored pattern image data, a scan line is seton an image signal (on the data stored in the storage unit 20), the scanis carried out along the scan line, and the data required for thedetection of an angle or the detection of a runout is obtained. Here,since the scan line is a virtual line set on the image data, anarbitrary number of the scan lines can be set at the arbitrarypositions, and increasing the number of the scan lines enables improvinga measurement accuracy.

FIG. 6 shows a relationship between the circular pattern 25, the angledetection pattern 26, the reference designation pattern 27, and the scanlines in the detection pattern image. In FIG. 6, X and Y represent an Xaxis and a Y axis set on the image sensor 12. An intersecting point ofthe X axis and the Y axis, i.e., an origin of an X-Y rectangularcoordinate coincides with the optical axis 9 b. Further, in FIG. 6, thedarkening of the line segments 26 a, the position designation pattern 27a and the direction designation patterns 27 b is omitted.

STEP 02 A center position of the detection pattern 11 is coarselydetected. The coarse detection of the center position is carried outbased on the circular pattern 25.

A predetermined range is set with the origin of the X-Y rectangularcoordinate as a reference, and a necessary number (three in the drawing)of the scan lines 37 a and 37 b is set so as to be parallel to the Xaxis and the Y axis. The scan is carried out along the scan lines 37 aand 37 b, and a signal of each pixel placed on the respective scan lines37 a and 37 b is acquired. Based on a signal of each pixel as acquired,the circular pattern 25 is detected.

By detecting the circular pattern 25, the center of the circle, i.e.,the center position of the detection pattern 11 is obtained as acoordinate value in the X-Y rectangular coordinate.

The circular scan lines 38 a, 38 b, and 38 c, and a scan line 39 are setwith the obtained center position at the center.

The scan lines 38 a, 38 b, and 38 c are the concentric circles, set onthe angle detection pattern 26, and have radii meeting R1>R2>R3, and R1,R2, and R3 are known values, respectively. It is to be noted that thenumber of the scan line 38 is set to be three, but is appropriatelydecided in accordance with a required accuracy. Furthermore, the scanline 39 is set on the reference designation pattern 27.

STEP 03 The scan line 39 is scanned in regard to the referencedesignation pattern 27, and signals are acquired in relation to the scanline 39. The acquired signals are shown in FIG. 7B. When the scan line39 is scanned, a position of the reference designation pattern 27, i.e.,a position of the position designation pattern 27 a is obtained. Theposition of the reference designation pattern 27 may be acquired as apolar coordinate, or may be converted into the X-Y rectangularcoordinate and then acquired.

STEP 04, STEP 05 In regard to the angle detection pattern 26, the totalcircumferential scanning is carried out on the scan lines 38 a, 38 b,and 38 c, and signals are obtained with respect to the respective scanlines 38 a, 38 b, and 38 c.

The signals obtained by scanning the scan lines 38 a, 38 b, and 38 c areshown in FIG. 7A. In regard to the scan, the scan line is changed everyrotation (360°), and the scan line is changed from the scan line 38 a tothe scan line 38 b and from the scan line 38 b to the scan line 38 c. Bythe fact that the total circumferential scanning is carried out on therespective scan lines 38 a, 38 b, and 38 c and the obtained results areaveraged, errors are cancelled out, and a highly accurate phase ismeasured.

STEP 06 Subsequently, a region with respect to the angle detectionpattern 26 is set. It is to be noted that the region may be setsimultaneously with setting the scan lines 38 a, 38 b, and 38 c.

FIG. 8 shows an example of the region setting.

With a center position obtained by the coarse detection in STEP 02 as areference, the angle detection pattern 26 is equally divided into aneven number (divided into four in the drawing) in the circumferentialdirection. The divided portions belonging to a range of 180° which isone of portions as divided by two are determined as A1 and A2, thedivided portions belonging to another range of 180° as B1 and B2.Further, the divided portion A1 and the divided portion B1 are arrangedto face each other, and the divided portion A2 and the divided portionB2 are arranged to face each other. Therefore, the phases of the dividedportions corresponding to each other are 180° different from each other.Each of the above-described divided portions is determined as oneregion.

STEP 07 First, the total circumferential scanning is carried out alongthe scan line 38 a, and in the obtained signals, signals of the linesegments 26 a belonging to each divided portion are acquired.

When the average waveform signals of frequencies of the divided portionsobtained from all the scan lines 38 a, 38 b, and 38 c are furtheraveraged, a highly accurate waveform can be obtained.

STEP 08 In regard to the divided portions as described above, when thewaveform and the phase as averaged are acquired, an averaged centerposition of the divided portion can be acquired with respect to eachdivided portion. Furthermore, when a polar coordinate of the centerposition of the divided portion is subjected to the coordinatetransformation, a coordinate position of the center position of thedivided portion in the X-Y rectangular coordinate can be obtained.

For example, referring to FIG. 8, the center position on the X-Yrectangular coordinate of each divided portion, i.e., each of thedivided portion A1, the divided portion B1, the divided portion A2, andthe divided portion B2 can be obtained respectively by calculation.

A straight line (Y center line) connecting the center of the dividedportion A1 to the center of the divided portion B1 and a straight line(X center line) connecting the center of the divided portion A2 to thecenter of the divided portion B2 can be obtained. A center position ofthe X center line (a midpoint of the X center line) and a centerposition of the Y center line (a midpoint of the Y center line) become acenter position of the detection pattern 11, respectively, and acoordinate position on the rectangular coordinate (a coordinate positionon the X-Y rectangular coordinate system) is calculated.

The obtained highly accurate center position of the detection pattern 11is stored in the storage unit 20 as an initial center coordinate.

It is to be noted that the X center line and the Y center line arerepresented by the following expressions.X center line=[Φ(A1)−Φ(B1)]/2Y center line=[Φ(A2)−Φ(B2)]/2

When the initial reference position, the initial waveform, the initialphase, and the initial center position in the initial state areacquired, a rotation angle of the rotation shaft 1 is measured.

The detection of the rotation angle will now be described with referenceto FIG. 5.

STEP 31, STEP 32 In a state that the rotation shaft 1 is rotated anecessary angle, STEP 01 to STEP 08 are executed, and the data after therotation, i.e., a reference position, a waveform, a phase, and a centerposition of the detection pattern 11 are obtained.

STEP 33 A coarse rotation angle is obtained from the initial referenceposition and a post-rotation reference position of the referencedesignation pattern 27. The coarse rotation angle can be obtained bymultiplying a number N of the line segments 26 a present between theinitial reference position and the post-rotation reference position(i.e., a number of frequencies present between the initial referenceposition and the post-rotation reference position) by 360°/n.

STEP 34 A precise rotation angle is obtained based on the initial phaseand the post-rotation phase. A phase difference σ is obtained from theinitial phase and the post-rotation phase. When this phase difference σis multiplied by 360°/n, an angle deviation in accordance with the phasedifference can be obtained. Therefore, the precise rotation angle is asfollows:N×360°/n+σ×360°/n

STEP 35 By comparing the coordinate of the initial center position ofthe detection pattern 11 is compared with the coordinate of thepost-rotation center position of the detection pattern 11, a deviationamount and a deviation direction can be calculated, and by correctingthe measured rotation angle based on the deviation amount and thedeviation direction as obtained, highly accurate angle measurement canbe performed while taking the eccentricity of rotation and the runoutinto consideration. That is, even if the rotation shaft 1 has therunout, the highly accurate angle measurement can be executed.

If the center position in the initial state does not coincide with theoptical axis 9 b, i.e., the X-Y rectangular coordinate, previouslyacquiring the information of the runout when the rotation shaft 1 makesone revolution likewise enables performing the highly accurate anglemeasurement.

The rotation shaft 1 is rotated 360° at a predetermined angle pitch, andthe center position (the center coordinate) of the detection pattern 11at each angle pitch is obtained.

If the detection pattern 11 deviates from the optical axis 9 a or therotation shaft 1 has the runout with respect to the bearing holder 3,the center position of the detection pattern 11 does not coincide withthe optical axis 9 b, and a locus of the center position is a circle oran ellipse (which will be referred to as an eccentric circlehereinafter).

However, the obtained eccentric circle has the high reproducibilitybecause of the mechanism and, if a rotating position (a rotation angle)of the rotation shaft 1 is known, a direction and an amount of therunout can be accurately grasped based on a locus of the deviation.Therefore, if the eccentric circle as the correcting information isacquired in advance, a measured angle is corrected, and the highlyaccurate angle measurement can be performed even in case of a rotationshaft having the runout. That is, highly accurate adjustment of a shaftof the rotation shaft 1 is not needed, and a production cost can bereduced.

As described above, since the eccentric circle has the highreproducibility, once the data of the eccentric circle is obtained, STEP35 can be omitted.

The invention claimed is:
 1. A rotation angle detecting apparatus,comprising a shaft portion space formed in a rotation shaft, a bearingholder space formed in a bearing holder, a first condenser lensaccommodated in said shaft portion space, a second condenser lensprovided in said bearing holder space and provided to face said firstcondenser lens, a detection pattern provided at a focal position of oneof said first condenser lens and said second condenser lens, an imagesensor provided at a focal position of the other of said first condenserlens and said second condenser lens, and an arithmetic unit forcalculating an angle displacement of said rotation shaft based on asignal from said image sensor, wherein said detection pattern has anangle detection pattern in which bar-like line segments extending in aradial direction are arranged at a predetermined angle pitch and aring-like track is formed by bar-like line segments, and a referencedesignation pattern for indicating a reference position of said angledetection pattern, and said arithmetic unit carries out a totalcircumferential scanning on said track with a predetermined radius witha center of said angle detection pattern as the center in regard to aprojection image of said detection pattern obtained by said imagesensor, extracts a frequency component, carries out a scanning of saidreference designation pattern, and calculates said angle displacement ofsaid rotation shaft based on a phase difference of said frequencycomponent and the number of frequencies corresponding to a change inposition of said reference designation pattern.
 2. The rotation angledetecting apparatus according to claim 1, wherein said line segments ofsaid angle detection pattern are obtained by dividing said track by 2nradii at an equal angle pitch and have a wedge-like shape.
 3. Therotation angle detecting apparatus according to claim 1, wherein saiddetection pattern has a circular pattern that is concentric with saidangle detection pattern, and said arithmetic unit scans a projectionimage of said circular pattern in two directions orthogonal to eachother and obtains said center of said angle detection pattern.
 4. Therotation angle detecting apparatus according to claim 1, wherein saidtrack is equally divided into an even number, an average of phases offrequency components of said respective line segments is obtained ineach divided portion, and said center of said angle detection pattern isdetected from a phase difference between said divided portions that faceeach other.
 5. The rotation angle detecting apparatus according to claim1, wherein said total circumferential scanning is carried out more thanonce with different radii.
 6. The rotation angle detecting apparatusaccording to claim 2, wherein said total circumferential scanning iscarried out more than once with different radii.
 7. The rotation angledetecting apparatus according to claim 3, wherein said totalcircumferential scanning is carried out more than once with differentradii.
 8. The rotation angle detecting apparatus according to claim 4,wherein said total circumferential scanning is carried out more thanonce with different radii.